Cat-ear power supply having a latch reset circuit

ABSTRACT

A cat-ear power supply is operable to generate a DC voltage and draws current from an AC power source near the beginning and end of a half-cycle of the AC power source. A controllably conductive switching circuit selectively charges an energy storage capacitor to produce the DC voltage and become conductive to charge the energy storage capacitor near the beginning of the half-cycle of the AC power source. A latch circuit controls the controllably conductive switching circuit to become non-conductive in response to the magnitude of the DC voltage. A switch voltage monitor circuit controls the controllably conductive switching circuit to become non-conductive and resets the latch circuit when the magnitude of a switch voltage across the switching circuit exceeds a predetermined switch voltage threshold. The switching circuit becomes conductive to charge the energy storage capacitor near the end of the half-cycle when the magnitude of the switch voltage drops below the predetermined switch voltage threshold.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power supply for a load controldevice, and more particularly, to a cat-ear power supply that is able tocharge an energy storage capacitor from an alternating-current (AC)power source at the beginning and the end of each half-cycle of the ACpower source.

2. Description of the Related Art

A conventional two-wire dimmer has two connections: a “hot” connectionto an alternating-current (AC) power supply and a “dimmed hot”connection to the lighting load. Standard dimmers use one or moresemiconductor switches, such as triacs or field effect transistors(FETs), to control the current delivered to the lighting load and thusto control the intensity of the lighting load. The semiconductorswitches are typically coupled between the hot and dimmed hotconnections of the dimmer.

Smart wall-mounted dimmers may include a user interface typically havinga plurality of buttons for receiving inputs from a user and a pluralityof status indicators for providing feedback to the user. These smartdimmers typically include a microprocessor or other processing devicefor allowing an advanced set of control features and feedback options tothe end user. An example of a smart dimmer is disclosed in commonlyassigned U.S. Pat. No. 5,248,919, issued on Sep. 28, 1993, entitledLIGHTING CONTROL DEVICE, which is herein incorporated by reference inits entirety.

In order to provide a direct-current (DC) voltage V_(CC) to power themicroprocessor and other low-voltage circuitry, the smart dimmerstypically include cat-ear power supplies. A cat-ear power supply drawscurrent only near the beginning and the end of a half-cycle of the ACsource voltage and derives its name from the shape of the currentwaveform that it draws from the AC voltage source. Because the smartdimmer only has two terminals, the power supply must draw currentthrough the connected lighting load. In order for the power supply to beable to draw sufficient current, the semiconductor switch must benon-conductive so that a sufficient voltage is available across thepower supply. Thus, the semiconductor cannot be turned on for the entirelength of a half-cycle, even when the maximum voltage across thelighting load is desired.

FIG. 1 is a simplified schematic diagram of a prior art cat-ear powersupply 10. The power supply 10 comprises an energy storage capacitor C12across which a DC supply voltage V_(C) is produced. The power supply 10also comprises a full-wave bridge rectifier BR14 that is coupled to anAC power source and produces a rectified voltage V+. The capacitor C12is operable to charge through the rectifier BR14, a switching circuit20, and a diode D16 to generate the supply voltage V_(C).

The switching circuit 20 comprises a semiconductor switch (e.g., a FETQ22), having drain and source terminals coupled in series electricalconnection between the rectifier BR14 and the capacitor C12. A gate ofthe FET Q22 is coupled to a drive source circuit 30 through resistorsR24, R26, R28. The drive source circuit 30 generates a drive voltage,which is used to control the FET Q22 into the conductive state. Toproduce the drive voltage, a capacitor C32 charges from the rectifiedvoltage V+ through a resistor R34 and a diode D36. During eachhalf-cycle of the AC power source, the capacitor C32 begins to chargewhen the magnitude of the AC voltage exceeds the voltage across theseries combination of the capacitor C32 and the capacitor C12. Thevoltage across the capacitor C32 is limited to approximately a breakovervoltage of a zener diode Z38 (e.g., approximately 40 V). When thevoltage at the gate of the FET Q22 exceeds a predetermined gate voltage(e.g., approximately 15 volts), the FET is rendered conductive allowingthe capacitor C12 to charge. A zener diode Z29 of the switching circuit20 prevents the voltage at the gate of the FET Q22 from exceeding apredetermined safe operating voltage (e.g., 25 V) to protect the FET.

An overcurrent protection circuit 40 is coupled in series between theswitching circuit 20 and the capacitor C12. A sense voltage generatedacross a sense resistor R42 is representative of the magnitude of thecurrent through the capacitor C12. If the current through the senseresistor R42 exceeds a predetermined current limit, an NPN bipolarjunction transistor Q44 is rendered conductive. Accordingly, the gate ofthe FET Q22 is pulled down towards circuit common, such that the FET isrendered non-conductive.

A turn-off circuit 50 is coupled to the junction of the resistors R26,R28 of the switching circuit 20 and is operable to render the FET Q22non-conductive when either the magnitude of the voltage across thecapacitor C12 reaches a predetermined peak supply voltage level or themagnitude of the AC line voltage exceeds a predetermined line voltagelevel. Specifically, the junction of the overcurrent protection circuit40 and the diode D16 is coupled to the base of an NPN transistor Q52through a resistor R62 and a zener diode Z64. When the magnitude of thevoltage across the capacitor C12 and the diode D16 is such that thevoltage across the zener diode Z64 is greater than the breakover voltageof the zener diode, the zener diode begins to conduct current into thebase of the transistor Q52. Accordingly, the transistor Q52 is renderedconductive and the gate of the FET Q22 is pulled down towards circuitcommon, thus, rendering the FET non-conductive. The base of thetransistor Q52 is also coupled to a line voltage detect circuit 70,which comprises a voltage divider having two resistor R72, R74. When themagnitude of the AC line voltage exceeds the predetermine line voltagelevel, the transistor Q52 is rendered conductive, the drain-sourceimpedance of the FET Q22 increases, and the magnitude of the currentthrough the capacitor C12 is limited.

After the transistor Q52 is rendered conductive during a half-cycle ofthe AC power source, a latch circuit 80 prevents the FET Q22 from beingrendered conductive until near the end of the present half-cycle. Thelatch circuit 80 comprises a PNP transistor Q82 having a base coupled tothe collector of the transistor Q52 through a resistor R84. When thetransistor Q52 becomes conductive, the base of the transistor Q82 ispulled down towards circuit common and the transistor Q82 is renderedconductive. Accordingly, the transistor Q82 conducts current through aresistor R86 and into the base of the transistor Q52 to maintain thetransistor Q52 conductive until the end of the half-cycle.

Thus, the capacitor C12 is limited to charging only at the beginning ofeach half-cycle. There is a need for a power supply that charges at thebeginning of each half-cycle, and latches off to prevent the energystorage capacitor from charging near the peak of the AC line voltage,but also allows the energy storage capacitor to charge before the end ofthe half-cycle when the magnitude of the AC line voltage is below apredetermined magnitude.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, a cat-ear powersupply for a load control device, which controls the amount of powerdelivered to an electrical load from an AC power source, is operable togenerate a DC voltage and draw current from the AC power source near thebeginning and the end of a half-cycle of the AC power source. The powersupply comprises an energy storage capacitor, a controllably conductiveswitching circuit, a latch circuit, and a switch voltage monitorcircuit. The controllably conductive switching circuit is coupled inseries electrical connection with the energy storage capacitor forselectively charging the energy storage capacitor to produce the DCvoltage. The controllably conductive switching circuit is operable tobecome conductive to charge the energy storage capacitor near thebeginning of the half-cycle of the AC power source. A switch voltage isproduced across the controllably conductive switching circuit and isrepresentative of a magnitude of an AC line voltage of the AC powersource. The latch circuit is coupled to the controllably conductiveswitching circuit, wherein the latch circuit controls the controllablyconductive switching circuit to become non-conductive in response to themagnitude of the DC voltage. The switch voltage monitor circuit iscoupled to the controllably conductive switching circuit and to thelatch circuit, such that the switch voltage monitor circuit controls thecontrollably conductive switching circuit to become non-conductive, andalso resets the latch circuit when the magnitude of the switch voltageexceeds a predetermined switch voltage threshold. The controllablyconductive switching circuit is operable to become conductive to chargethe energy storage capacitor near the end of the half-cycle when themagnitude of the switch voltage drops below the predetermined switchvoltage threshold.

According to another embodiment of the present invention, a power supplyfor generating a DC voltage in a load control device for controlling theamount of power delivered to an electrical load from an AC power source,comprises: (1) an energy storage capacitor; (2) a controllablyconductive switching circuit coupled in series electrical connectionwith the energy storage capacitor for selectively charging the energystorage capacitor to produce the DC voltage, the controllably conductiveswitching circuit operable to become conductive to charge the energystorage capacitor near the beginning of a half-cycle of the AC powersource; (3) a latch circuit coupled to the controllably conductiveswitching circuit, wherein the latch circuit controls the controllablyconductive switching circuit to become non-conductive in response to themagnitude of the DC voltage; and (4) a switch voltage monitor circuitcoupled to the controllably conductive switching circuit, such that theswitch voltage monitor circuit controls the controllably conductiveswitching circuit to become non-conductive when the magnitude of the ACline voltage exceeds a predetermined line voltage threshold;characterized in that the switch voltage monitor circuit is furthercoupled to the latch circuit such that the latch circuit is reset whenthe magnitude of the AC line voltage exceeds the predetermined linevoltage threshold, and the controllably conductive switching circuit isoperable to become conductive to charge the energy storage capacitornear the end of the half-cycle when the magnitude of the AC line voltagedrops below the predetermined line voltage threshold.

A load control device for controlling an amount of power delivered to anelectrical load from an AC power source is also described herein. Theload control device comprises a controllably conductive device, acontroller, and a cat-ear power supply operable to generate a DC voltageand draw current from the AC power source near the beginning and the endof a half-cycle of the AC power source. The controllably conductivedevice is adapted to be coupled in series electrical connection betweenthe source and the load and has a control input for controlling thecontrollably conductive device between a non-conductive state and aconductive state. The controller is operatively coupled to the controlinput of the controllably conductive device for controlling thesemiconductor switch between the non-conductive state and the conductivestate. The cat-ear power supply comprises a controllably conductiveswitching circuit for selectively charging an energy storage capacitorto produce the DC voltage. The controllably conductive switching circuitis operable to become conductive to charge the energy storage capacitornear the beginning of a half-cycle of the AC power source. The powersupply further comprises a latch circuit for controlling thecontrollably conductive switching circuit to become non-conductive inresponse to the magnitude of the DC voltage. The switch voltage monitorcircuit controls the controllably conductive switching circuit to becomenon-conductive and resets the latch circuit when the magnitude of aswitch voltage across the controllably conductive device exceeds apredetermined switch voltage threshold. The controllably conductiveswitching circuit is operable to become conductive to charge the energystorage capacitor near the end of the half-cycle when the magnitude ofthe switch voltage drops below the predetermined switch voltagethreshold.

Other features and advantages of the present invention will becomeapparent from the following description of the invention that refers tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, there is shown in thedrawings a form, which is presently preferred, it being understood,however, that the invention is not limited to the precise arrangementsand instrumentalities shown. The features and advantages of the presentinvention will become apparent from the following description of theinvention that refers to the accompanying drawings, in which:

FIG. 1 is a simplified schematic diagram of a prior art cat-ear powersupply;

FIG. 2 is a simplified block diagram of a dimmer switch having a cat-earpower supply according to an embodiment of the present invention;

FIG. 3 is a simplified schematic diagram of the cat-ear power supply ofFIG. 2; and

FIG. 4 is a simplified diagram of waveforms that demonstrate theoperation of the power supply of FIG. 3.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The foregoing summary, as well as the following detailed description ofthe preferred embodiments, is better understood when read in conjunctionwith the appended drawings. For the purposes of illustrating theinvention, there is shown in the drawings an embodiment that ispresently preferred, in which like numerals represent similar partsthroughout the several views of the drawings, it being understood,however, that the invention is not limited to the specific methods andinstrumentalities disclosed.

FIG. 2 is a simplified block diagram of a load control device, e.g., adimmer switch 100, having a cat-ear power supply 200 according to anembodiment of the present invention. The dimmer switch 100 is coupled inseries electrical connection between an AC power source 102 (e.g., 120VAC @ 60 Hz) and an electrical load (e.g., a lighting load 104), suchthat the dimmer switch receives an AC line voltage V_(AC) from the ACpower source and controls of the amount of power delivered to thelighting load. The dimmer switch 100 comprises a hot terminal H coupledto the hot side of the AC power source 102 and a dimmed-hot terminal DHcoupled to the lighting load 104. The dimmer switch 100 furthercomprises a neutral terminal N coupled to the neutral side of the ACpower source 102.

The dimmer switch 100 comprises a controllably conductive device, e.g.,two FETs Q110, Q112 coupled in anti-series connection, for control ofthe amount of power delivered to the lighting load 104, and thus, theintensity of the lighting load. Alternatively, the controllablyconductive device could be implemented as any suitable type ofbidirectional semiconductor switch, such as, for example, a triac, twosilicon-controller rectifiers (SCRs) in anti-parallel connection, or aFET in a rectifier bridge. As shown in FIG. 2, the FETs Q110, Q112 arecoupled in series between hot terminal H and the dimmed-hot terminal DH,such that the FETs are adapted to be coupled between the source 102 andthe load 104. The junction of the FETs Q110, Q112 is coupled to acircuit common. The FETs Q110, Q112 may both comprise, for example, partnumber FDPF2710T, manufactured by Fairchild Semiconductors.

A controller 114 is coupled to control inputs (i.e., gates) of the FETsQ110, Q112 via first and second gate drive circuits 116, 118,respectively. The inputs provided to the gates cause the FETs Q110, Q112to be rendered conductive or non-conductive, which in turn controls thepower provided to the lighting load 104. To conduct load current fromthe AC power source 102 to the lighting load 104, the controller 114renders both FETs Q110, Q112 conductive. During the positivehalf-cycles, the controller 114 renders the first FET Q110non-conductive to stop the conduction of (i.e., block) the load current.During the negative half-cycles, the controller 114 renders the secondFET Q112 non-conductive to block the load current. The controller 114may be implemented as a microcontroller, a microprocessor, aprogrammable logic device (PLD), an application specific integratedcircuit (ASIC), a field-programmable gate array (FPGA), or any suitableprocessing device.

The controller 114 receives inputs from one or more control actuators120, which may comprise, for example, a toggle actuator and an intensityadjustment actuator. The controller 114 is operable to control theintensity of the lighting load 104 to a desired lighting intensity inresponse to the inputs received from the control actuators 120. Thecontroller 114 is further operable to illuminate one or more visualindicators 122, e.g., light-emitting diodes (LEDs), to display feedbackto a user. For example, the visual indicators 122 may be illuminated todisplay a representation of the amount of power presently beingdelivered to the lighting load 104.

A zero-crossing detector 124 is coupled between the hot terminal H andthe neutral terminal N and determines the zero-crossings of the input ACwaveform from the AC power supply 102. A zero-crossing is defined as thetime at which the AC supply voltage transitions from positive tonegative polarity, or from negative to positive polarity, at thebeginning of each half-cycle. The controller 114 provides the controlinputs to the drive circuits 116, 118 to operate the FETs Q110, Q112 atpredetermined times relative to the zero-crossing points of the ACwaveform.

The controller 114 may use forward-phase control or reversephase-control dimming techniques for control of the power delivered tothe lighting load 104. With forward phase control dimming, the FETsQ110, Q112 are rendered conductive at a specific time each half-cycle inresponse to the desired intensity of the lighting load 104, and the FETsQ110, Q112 are maintained conductive until the next voltagezero-crossing. The forward phase control dimming technique is often usedto control energy to a resistive or inductive load, which may include,for example, an incandescent lamp or a magnetic low-voltage transformer.In reverse phase control dimming, the FETs Q110, Q112 are controlled tobe conductive immediately following a zero-crossing of the AC linevoltage V_(AC) and then rendered non-conductive at a specific time laterin the half-cycle depending upon the desired intensity of the lightingload 104. The reverse phase control dimming technique is often used tocontrol energy to a capacitive load, which may include, for example, anelectronic low-voltage transformer.

The cat-ear power supply 200 is coupled between the neutral terminal Nand circuit common and generates a DC bus voltage V_(BUS) (e.g.,approximately 80 V). The cat-ear power supply 200 is operable to draw acharging current I_(CHRG) from the AC power source 102 through theneutral terminal N and the body diode of the first FET Q110 during thenegative half-cycles. The bus voltage V_(BUS) is received by asecond-stage low-voltage power supply 126 that generates a low-voltageDC supply voltage V_(CC) (e.g., 5 V) for powering the controller 114 andother low voltage circuitry of the dimmer switch 100.

FIG. 3 is a simplified schematic diagram of the cat-ear power supply200. FIG. 4 is a simplified diagram of waveforms that demonstrate theoperation of the power supply 200. The power supply 200 comprises anenergy storage capacitor C210, which is operable to conduct the chargingcurrent I_(CHRG) from the neutral terminal N through a first diode D212,a controllably conductive switching circuit 220, a second diode D214,and the energy storage capacitor C210. The bus voltage V_(BUS) isgenerated across the energy storage capacitor C210. The switchingcircuit 220 comprises a FET Q222 coupled in series electrical connectionwith the energy storage capacitor C210 for selectively charging theenergy storage capacitor to produce the bus voltage V_(BUS). The gate ofthe FET Q222 is coupled to the neutral terminal N through the firstdiode D212 and three resistors R224, R226, R228, which have, forexample, resistances of 10Ω, 15 kΩ, and 100 kΩ, respectively. Shortlyafter the beginning of each half-cycle (e.g., at time t₀ in FIG. 4), theFET Q222 is rendered conductive, the charging current I_(CHRG) begins toflow, and the capacitor C210 begins to charge when an appropriate gatevoltage (e.g., 3 V) is provided at the gate of the FET Q222. A zenerdiode Z229 is coupled to the gate of the FET Q222 to prevent the gatevoltage from exceeding a predetermined maximum gate voltage (e.g., 12 V)and damaging the FET. The FET Q222 may comprise, for example, partnumber STN1NK60, manufactured by ST Microelectronics.

An overcurrent protection circuit 230 is coupled in series with the FETQ222 and the energy storage capacitor C210. A sense voltage generatedacross a sense resistor R232 (which has, for example, a resistance of 1Ω) is representative of the magnitude of the current through the energystorage capacitor C210. A resistive divider (having resistors R234,R236) is coupled across the sense resistor R232. The junction of the tworesistors R234, R236 is coupled to the base of an NPN bipolar junctiontransistor Q238. For example, the resistors R234, R236 may haveresistances of 2.2 kΩ and 69.2 kΩ, respectively, such that thetransistor Q238 is rendered conductive when the current through thesense resistor R232 exceeds a predetermined overcurrent threshold (e.g.,0.6 A). Accordingly, when the transistor Q238 is conductive, themagnitude of the voltage at the gate of the FET Q222 decreases and thedrain-source impedance of the FET Q222 increases, thus limiting themagnitude of the charging current I_(CHRG).

A voltage regulation circuit 240 is coupled across the energy storagecapacitor C210 and is responsive to the magnitude of the bus voltageV_(BUS). The voltage regulation circuit 240 is coupled to a latchcircuit 250 and is operable to cause the latch circuit to control thecontrollably conductive switching circuit 220 to become non-conductivewhen the magnitude of the bus voltage V_(BUS) exceeds a predeterminedpeak voltage threshold V_(PV-TH) (e.g., at time t₁ in FIG. 4). Thevoltage regulation circuit 240 comprises a semiconductor switch (e.g.,an NPN bipolar junction transistor Q242) and a resistive divider (havingresistors R244, R246) coupled across the capacitor C210. The junction ofthe resistors R244, R246 is coupled to the base of the transistor Q242.The transistor Q238 of the overcurrent protection circuit 230 and thetransistor Q242 of the voltage regulation circuit 240 may comprise adual-transistor part, e.g., part number MMDT3904, manufactured byDiodes, Inc. The resistors R244, R246 may have resistances of, forexample, 100 Ω and 56 kΩ, respectively.

A zener diode Z248 is coupled between the emitter of the transistor Q242and circuit common and has, for example, a breakover voltage ofapproximately 91 V. When the magnitude of the bus voltage exceeds apredetermined peak voltage level (e.g., 91 V), the transistor Q242 isrendered conductive causing the latch circuit to control thecontrollably conductive switching circuit 220 to become non-conductiveas will be described in greater detail below.

The latch circuit 250 comprises a first semiconductor switch (e.g., aPNP bipolar junction transistor Q252) coupled in series electricalconnection with a first resistor R254 (e.g., having a resistance of 10kΩ). The latch circuit 250 further comprises a second semiconductorswitch (e.g. an NPN bipolar junction transistor Q256) coupled in serieselectrical connection with a second resistor R258 (e.g., having aresistance of 10 kΩ). The series combination of the first transistorQ252 and the first resistor R254 is coupled in parallel electricalconnection with the series combination of the second transistor Q256 andthe second resistor R258. The first transistor Q252 has a base coupledto the junction of the collector of the second transistor Q256 and thesecond resistor R258, while the second transistor Q256 has a basecoupled to the junction of the collector of the first transistor Q252and the first resistor R254. The base of the first transistor Q252 isfurther coupled to the collector of the transistor Q242 of the voltageregulation circuit 240. The first and second transistors Q252, Q256 maycomprise a dual-transistor part, e.g., part number MMDT3946,manufactured by Diodes, Inc.

When the magnitude of the bus voltage V_(BUS) exceeds the predeterminedpeak voltage threshold V_(PV-TH) and the transistor Q242 of the voltageregulation circuit 240 is rendered conductive, the voltage across thelatch circuit is reduced to and maintained at substantially zero volts,e.g., less than or equal to approximately 1.4 volts. Specifically, whenthe transistor Q242 is rendered conductive, a current is drawn throughthe second resistor R258 and the first transistor Q252 also becomesconductive. The first transistor Q252 then conducts a current throughthe first resistor R254 and the second transistor Q256 is renderedconductive, thus, latching the transistors Q252, Q256 both in theconductive state. When both of the transistors Q252, Q256 areconductive, the voltage across the latch circuit 250 is substantiallyzero volts and the voltage at the gate of the FET Q222 is reduced, thus,rendering the FET Q222 non-conductive. The FET Q222 is maintainednon-conductive as long as the latch circuit 250 stays latched.

A switch voltage monitor circuit 260 is coupled to the controllablyconductive switching circuit 220 and the latch circuit 250. The switchvoltage monitor circuit 260 renders the FET Q222 non-conductive when themagnitude of a switch voltage V_(SW) (across the controllably conductiveswitching circuit 220 and the overcurrent protection circuit 230)exceeds a predetermined switch voltage threshold V_(SW-TH) (e.g.,approximately 70 V). The magnitude of the switch voltage V_(SW) isrepresentative of the magnitude of the AC line voltage V_(AC) during thenegative half-cycles. Specifically, the magnitude of the switch voltageV_(SW) is approximately equal to the magnitude of the AC line voltageV_(AC) subtracted by the magnitude of the bus voltage V_(BUS) and theforward voltage drops of the diode D212, the diode D214, and the bodydiode of the first FET Q110.

The switch voltage monitor circuit 260 maintains the FET Q222 in thenon-conductive state as long as the magnitude of the switch voltageV_(SW) is above the predetermined switch voltage threshold V_(SW-TH).The switch voltage monitor circuit 260 further operates as a latch resetcircuit to reset the latch circuit 250 (such that the latch circuitbecomes unlatched) when the magnitude of the switch voltage V_(SW)becomes greater than the predetermined switch voltage thresholdV_(SW-TH) (e.g., at time t₂ in FIG. 4). Since the latch circuit 250 isno longer latched after the magnitude of the switch voltage V_(SW) hasexceeded the predetermined switch voltage threshold V_(SW-TH), the FETQ222 is operable to become conductive to charge the energy storagecapacitor C210 once again when the magnitude of the switch voltageV_(SW) drops below the predetermined switch voltage threshold V_(SW-TH)near the end of the half-cycle (e.g., at time t₃ in FIG. 4).

The switch voltage monitor circuit 260 comprises a semiconductor switch(e.g., an NPN bipolar junction transistor Q262), which is coupled acrossthe series combination of the first transistor Q252 and the firstresistor R254 of the latch circuit 250, i.e., across the latch circuit.A resistive divider (having two resistors R264, R266) is coupled betweenthe diode D212 and the emitter of the transistor Q252 to produce avoltage representative of the magnitude of the switch voltage V_(SW) atthe junction of the resistors R264, R266. A zener diode Z268 is coupledbetween the junction of the two resistors R264, R266 and the base of thetransistor Q252, and a resistor R269 is coupled between the base and theemitter of the transistor Q252. For example, the resistors R264, R266,R269 have resistances of 100 kΩ, 18 kΩ, and 100 kΩ, respectively, andthe zener diode Z268 has a breakover voltage of 10 V, such that when themagnitude of the switch voltage V_(SW) exceeds approximately 70 V, thezener diode Z268 begins to conduct current, and the transistor Q252 isrendered conductive. Accordingly, the FET Q222 is renderednon-conductive and the latch circuit 250 is reset. When the magnitude ofthe switch voltage V_(SW) decreases below the predetermined switchvoltage threshold V_(SW-TH) (i.e., 70 V), the transistor Q252 is becomesnon-conductive and the FET Q222 is operable to become conductive, thus,allowing the capacitor C210 to charge near the end of the half-cycle.

The cat-ear power supply 200 is described herein as being coupledbetween the neutral terminal and circuit common such that power supplycharges during the negative half-cycles. One skilled in the art willeasily recognize that the concepts of the present invention could beapplied to a cat-ear power supply that charges only during the positivehalf-cycles or during both the positive and negative half-cycles.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art.Therefore, the present invention should not be limited by the specificdisclosure herein.

1. A cat-ear power supply for a load control device for controlling theamount of power delivered to an electrical load from an AC power source,the power supply operable to generate a DC voltage, the power supplycomprising: an energy storage capacitor; a controllably conductiveswitching circuit coupled in series electrical connection with theenergy storage capacitor for selectively charging the energy storagecapacitor to produce the DC voltage, the controllably conductiveswitching circuit operable to become conductive to charge the energystorage capacitor near the beginning of a half-cycle of the AC powersource; a latch circuit coupled to the controllably conductive switchingcircuit, wherein the latch circuit controls the controllably conductiveswitching circuit to be non-conductive in response to the magnitude ofthe DC voltage; and a switch voltage monitor circuit coupled to thecontrollably conductive switching circuit and the latch circuit, suchthat the switch voltage monitor circuit controls the controllablyconductive switching circuit to be non-conductive when the magnitude ofa switch voltage across the controllably conductive switching circuitexceeds a predetermined switch voltage threshold and also resets thelatch circuit when the magnitude of the switch voltage exceeds thepredetermined switch voltage threshold; wherein the controllablyconductive switching circuit is operable to become conductive to chargethe energy storage capacitor near the end of the half-cycle when themagnitude of the switch voltage drops below the predetermined switchvoltage threshold.
 2. The power supply of claim 1, wherein the switchvoltage monitor circuit comprises a switch voltage monitor semiconductorswitch coupled across the latch circuit, such that the voltage acrossthe latch circuit is controlled to substantially zero volts when themagnitude of the switch voltage exceeds the predetermined switch voltagethreshold.
 3. The power supply of claim 2, further comprising: a voltageregulation circuit coupled to the latch circuit and the energy storagecapacitor, and operable to cause the latch circuit to control thecontrollably conductive switching circuit to become non-conductive whenthe magnitude of the DC voltage exceeds a predetermined peak voltagethreshold.
 4. The power supply of claim 3, wherein the latch circuitcomprises a first latch semiconductor switch coupled in serieselectrical connection with a first resistor, the switch voltage monitorsemiconductor switch of the switch voltage monitor circuit coupled inparallel with the series combination of the first latch semiconductorswitch and the first resistor, the first latch semiconductor switchrendered conductive when the magnitude of the DC voltage exceeds thepredetermined peak voltage threshold.
 5. The power supply of claim 4,wherein the latch circuit comprises a second latch semiconductor switchcoupled in series electrical connection with a second resistor, theseries combination of the second latch semiconductor switch and thesecond resistor coupled in parallel with the series combination of thefirst latch semiconductor switch and the first resistor, the secondlatch semiconductor switch rendered conductive in response to the firstlatch semiconductor switch being rendered conductive.
 6. The powersupply of claim 5, wherein the first latch semiconductor switchcomprises a PNP bipolar junction transistor having a base coupled to thejunction of the second latch semiconductor switch and the secondresistor, the second latch semiconductor switch comprising an NPNbipolar junction transistor having a base coupled to the junction of thefirst latch semiconductor switch and the first resistor, the base of thefirst latch semiconductor switch further coupled to the voltageregulation circuit, such that the voltage across the latch circuit ismaintained at a substantially low voltage after the magnitude of the DCvoltage exceeds a predetermined peak voltage threshold.
 7. The powersupply of claim 2, wherein the switch voltage monitor semiconductorswitch comprises an NPN bipolar junction transistor, the switch voltagemonitor circuit further comprising a zener diode adapted to be coupledbetween the AC power source and a base of the transistor, such that thetransistor is rendered conductive when the voltage across the zenerdiode exceeds a breakover voltage of the zener diode.
 8. The powersupply of claim 1, wherein the controllably conductive switching circuitcomprises a semiconductor switch.
 9. The power supply of claim 8,wherein the semiconductor switch comprises a field-effect transistor.10. The power supply of claim 1, further comprising: a voltageregulation circuit coupled to the latch circuit and the energy storagecapacitor, and operable to cause the latch circuit to control thecontrollably conductive switching circuit to become non-conductive whenthe magnitude of the DC voltage exceeds a predetermined peak voltagethreshold.
 11. The power supply of claim 1, wherein the load controldevice is operable to receive an AC line voltage of the AC power source,and the magnitude of the switch voltage is representative of themagnitude of the AC line voltage.
 12. A load control device forcontrolling an amount of power delivered to an electrical load from anAC power source, the load control device comprising: a controllablyconductive device adapted to be coupled in series electrical connectionbetween the source and the load, the controllably conductive devicehaving a control input for controlling the controllably conductivedevice between a non-conductive state and a conductive state; acontroller operatively coupled to the control input of the controllablyconductive device for controlling the semiconductor switch between thenon-conductive state and the conductive state; a cat-ear power supplyfor generating a DC voltage, the power supply comprising a controllablyconductive switching circuit for selectively charging an energy storagecapacitor to produce the DC voltage, the controllably conductiveswitching circuit operable to be conductive to charge the energy storagecapacitor near the beginning of a half-cycle of the AC power source, thepower supply further comprising a latch circuit for controlling thecontrollably conductive switching circuit to be non-conductive inresponse to the magnitude of the DC voltage, and a switch voltagemonitor circuit for controlling the controllably conductive switchingcircuit to become non-conductive and also resetting the latch circuitwhen the magnitude of a switch voltage across the controllablyconductive switching circuit exceeds a predetermined switch voltagethreshold; wherein the controllably conductive switching circuit isoperable to become conductive to charge the energy storage capacitornear the end of the half-cycle when the magnitude of the switch voltagedrops below the predetermined switch voltage threshold.
 13. The loadcontrol device of claim 12, wherein the controllably conductive devicecomprises a bidirectional semiconductor switch.
 14. The load controldevice of claim 13, wherein the bidirectional semiconductor switchcomprises two FETs in anti-series connection.
 15. The load controldevice of claim 14, wherein the controller is operable to control theFETs using a reverse-phase control dimming technique.
 16. The loadcontrol device of claim 12, further comprising: a neutral terminalcoupled to the power supply, the neutral connection adapted to becoupled to the neutral side of the AC power source, such that the powersupply is operable to draw current from the AC power source through theneutral terminal.
 17. The load control device claim 12, wherein the loadcontrol device is operable to receive an AC line voltage of the AC powersource, and the magnitude of the switch voltage is representative of themagnitude of the AC line voltage.
 18. A power supply for a load controldevice for controlling the amount of power delivered to an electricalload from an AC power source, the load control device operable toreceive an AC line voltage of the AC power source, the power supplyoperable to generate a DC voltage, the power supply comprising: anenergy storage capacitor; a controllably conductive switching circuitcoupled in series electrical connection with the energy storagecapacitor for selectively charging the energy storage capacitor toproduce the DC voltage, the controllably conductive switching circuitoperable to become conductive to charge the energy storage capacitornear the beginning of a half-cycle of the AC power source; a latchcircuit coupled to the controllably conductive switching circuit,wherein the latch circuit controls the controllably conductive switchingcircuit to become non-conductive in response to the magnitude of the DCvoltage; and a switch voltage monitor circuit coupled to thecontrollably conductive switching circuit, such that the switch voltagemonitor circuit maintains the controllably conductive switching circuitnon-conductive when the magnitude of the AC line voltage exceeds apredetermined line voltage threshold; characterized in that the switchvoltage monitor circuit is further coupled to the latch circuit suchthat the latch circuit is reset when the magnitude of the AC linevoltage exceeds the predetermined line voltage threshold, and thecontrollably conductive switching circuit is operable to becomeconductive to charge the energy storage capacitor near the end of thehalf-cycle when the magnitude of the AC line voltage drops below thepredetermined line voltage threshold.